FIG. 10 is a cross-sectional view schematically showing a configuration for a conventional ultrasonic generator (which will also be referred to herein as a “piezoelectric vibrator”) 10. The ultrasonic generator 10 includes a case 1, a piezoelectric layer (vibrating means) 2 and an acoustic impedance matching layer (or matching means, which will be referred to herein as an “acoustic matching layer”). The case 1 and the piezoelectric layer 2 are bonded together by way of an adhesive layer of an epoxy adhesive, for example. The case 1 and the acoustic matching layer 100 are also connected together with a similar adhesive. The piezoelectric layer vibrates at about 500 kHz. The vibrations are transmitted to the case 1 by way of the adhesive layer (not shown) and then to the acoustic matching layer 100 by way of the second adhesive layer. Then, the vibrations of the acoustic matching layer 100 are propagated as acoustic waves into a gas that exists in the space. Although not described in detail for the sake of simplicity, two electrodes (not shown) are provided on the upper and lower surfaces of the piezoelectric layer 2 to polarize the piezoelectric layer 2 in the thickness direction. The ultrasonic generator 10 can convert electrical energy into mechanical energy, or vice versa, by using the piezoelectric layer 2. The case 1 includes a top plate 1a that defines a concave portion to embed the piezoelectric layer 2 therein and a bottom plate 1b that is arranged so as to close up the inner space of the concave portion hermetically. The piezoelectric layer 2 is included hermetically inside of the concave portion. One of the two electrodes on the two principal surfaces of the piezoelectric layer 2 is connected to a terminal 5a by way of the case 1, while the other electrode is connected to a terminal 5b. Accordingly, the case 1 is normally made of a metal with electrical conductivity.
It is the role of the acoustic matching layer 100 to propagate the vibrations of the piezoelectric layer 2 to the gas efficiently. The acoustic impedance Z of a substance is defined by the following Equation (1):Z=ρ×C  (1)where C is the sonic velocity in the substance and ρ is the density of the substance. The acoustic impedance Z of the piezoelectric layer 2 is significantly different from that of the gas.
Specifically, the acoustic impedance Z1 of a piezoelectric body that makes the piezoelectric layer 2 is 30×106 kg/s·m2, while the acoustic impedance Z3 of the gas (e.g., the air) is 4.28×102 kg/s·m2. The acoustic impedance of the piezoelectric body is approximately equal to that of a metal. In this manner, sound (i.e., a vibration) being propagated is reflected from the boundary surface between two substances with mutually different acoustic impedances. As a result, the sound transmitted has a decreased intensity. However, by inserting a substance with a third acoustic impedance between the two substances with mutually different acoustic impedances, such sound reflection can be reduced.
It is generally known that the sound reflection can be eliminated by inserting a substance, of which the acoustic impedance Z2 satisfies the following Equation (2), between the piezoelectric layer 2 and the space (i.e., the gas medium into which the acoustic wave is radiated):Z2=(Z1·Z3)1/2  (2)
If the acoustic impedances Z1 (=30×106 kg/s·m2) and Z3 (=4.28×102 kg/s·m2) described above are substituted into this equation, then the resultant Z2 value will be 0.11×106 kg/s·m2. A substance having such an acoustic impedance needs to be a solid with a low density and a low sonic velocity.
Even in a gas ultrasonic generator that uses a piezoelectric body of PZT, for example, a matching layer for matching the acoustic impedance of the piezoelectric body to that of the gas (e.g., the air) is normally provided on the vibrating surface in order to radiate the ultrasonic wave, generated in the PZT, to the gas as the propagating medium. In an ultrasonic transducer that radiates an ultrasonic wave, generated in PZT, into the air, the acoustic impedance Z1 (of about 400 kg/s·m2) of the air (i.e., the gas) is far smaller than (i.e., approximately one-hundred-thousandth of) the acoustic impedance Z3 (of about 30×105 kg/s·m2) of PZT (i.e., the solid). Thus, for such an ultrasonic transducer to radiate the ultrasonic wave efficiently, the acoustic impedance of the acoustic matching layer thereof is a key factor.
As a conventional acoustic matching layer to be provided on the vibrating surface of a piezoelectric layer (which will be sometimes referred to herein as an “piezoelectric vibrator”) made of a piezoceramic such as PZT (lead zirconate titanate), an acoustic matching layer 100 made of an epoxy resin 112, in which glass balloons (tiny hollow glass spheres) 110 are dispersed, is known as shown in FIG. 11. The acoustic matching layer 100 has its density decreased by solidifying the tiny glass balloons 110 with the epoxy resin 112. The glass balloons 110 have diameters of 100 μm or less, because the glass balloons 110 needs to be sufficiently smaller than the wavelength of the sound being propagated through the acoustic matching layer.
The intensity of the sound to be propagated into the gas after having been transmitted through the acoustic matching layer 100 is also changeable with the thickness of the acoustic matching layer 100 (i.e., the distance that the acoustic wave goes through the acoustic matching layer). The acoustic wave that has come from the piezoelectric layer 2 splits into a wave to be transmitted and a wave to be reflected from the boundary surface between the acoustic matching layer 100 and the gas. The reflected wave is further reflected from the boundary surface between the acoustic matching layer 100 and the piezoelectric layer 2 to have its phase inverted. Thereafter, a portion of this wave will be transmitted through the boundary surface between the acoustic matching layer 100 and the gas. The thickness t at which the transmittance T is maximized through the synthesis of these waves is given by t=λ/4.
When the acoustic matching layer 100 including the glass balloons 110 is used, the acoustic matching layer 100 has a sonic velocity of 2,000 m/s. Accordingly, if the sound has a frequency of 500 kHz, then the sound being propagated through the acoustic matching layer 100 has a wavelength λ of 4 mm. Thus, the best thickness t of the acoustic matching layer 100 is 1 mm.
If the thickness t of the acoustic matching layer is defined to be an integral number of times as large as λ/4, then a theoretical equation for calculating the transmittance T of an ultrasonic energy to be radiated from an ultrasonic vibrator into an external propagating medium where the acoustic matching layer is provided on the vibrating surface of the ultrasonic vibrator can be represented as the following simplified Equation (3):T=4·Z1·Z3·Z22/(Z1·Z3+Z22)2  (3)
The relationship between the transmittance T of an ultrasonic energy through the air and the acoustic impedance Z2 of the acoustic matching layer 100, made of the conventional epoxy resin with glass balloons, will be described. Specifically, the epoxy resin with glass balloons has an acoustic impedance of about 1.2×106 kg/s·m2, and Z22≈1.44×1012. In the example described above, Z1·Z3=400×1.2×106=4.8×108. Thus, Z1·Z3<<Z22. Accordingly, the Equation (3) described above is further approximated as T≈4·Z1·Z3/Z22. Consequently, it can be seen that the transmittance T of the ultrasonic energy is inversely proportional to the square of the acoustic impedance Z2 of the acoustic matching layer 100. That is to say, the smaller the acoustic impedance Z2 of the acoustic matching layer 100, the higher the transmittance T of the ultrasonic energy.
FIG. 9 schematically shows a configuration for an ultrasonic flowmeter including the ultrasonic generator 10 described above. In this example, a pair of ultrasonic generators 10 is used as a pair of ultrasonic transducers 101 and 102.
As shown in FIG. 9, the ultrasonic generators 101 and 102 are provided in the tube (or tube wall) 52 that defines the channel 51 of the gas. If the ultrasonic transducer 101 or 102 is broken, then the gas will leak out of the tube 52. For that reason, it is hard to choose an easily breakable material such as a ceramic or a resin as a material for the case (i.e., the case 1 shown in FIG. 9) of the ultrasonic transducers 101 and 102. Accordingly, a metal material such as stainless steel or iron is used as a material for the case.
Suppose a fluid is now flowing at a velocity V in the direction indicated by the bold arrow along a channel 51 as shown in FIG. 9. The ultrasonic transducers 101 and 102 are provided in the tube wall 52 so as to face each other. Each of the ultrasonic transducers 101 and 102 includes a piezoelectric vibrator, made of a piezoceramic, for example, as an electromechanical energy converter, and exhibits a resonance characteristic just like a piezoelectric buzzer or a piezoelectric oscillator. In this example, the ultrasonic transducer 101 is used as an ultrasonic transmitter and the ultrasonic transducer 102 is used as an ultrasonic receiver. A driver circuit 54, a reception sensing circuit 56, a timer 57, a calculating section 58, and a control section 59 are connected to the ultrasonic transducers 101 and 102 by way of a switching circuit 55 that switches the transmission and reception of the transducers. The driver circuit 54 drives the ultrasonic transducers 101 and 102. The reception sensing circuit 56 senses an ultrasonic pulse received. The timer 57 measures the propagation time of the ultrasonic pulse. The calculating section 58 calculates the flow rate based on the output of the timer 57. The control section 59 outputs a control signal to the driver circuit 54 and timer 57.
Hereinafter, it will be described how this ultrasonic flowmeter operates.
When an alternating current voltage, having a frequency in the vicinity of the resonant frequency, is applied to the piezoelectric layer of the ultrasonic transducer 101, the ultrasonic transducer 101 radiates an ultrasonic wave into the external fluid such that the ultrasonic wave goes along the propagation path L shown in FIG. 9. Then, the ultrasonic transducer 102 receives the ultrasonic wave propagated and transforms it into a voltage.
Thereafter, the ultrasonic transducer 102 is used as an ultrasonic transmitter and the ultrasonic transducer 101 is used as an ultrasonic receiver in turn. Specifically, when an alternating current voltage, having a frequency in the vicinity of the resonant frequency, is applied to the piezoelectric layer of the ultrasonic transducer 102, the ultrasonic transducer 102 radiates an ultrasonic wave into the external fluid such that the ultrasonic wave goes along the propagation path L shown in FIG. 9. Then, the ultrasonic transducer 101 receives the ultrasonic wave propagated and transforms it into a voltage. In this manner, each of the ultrasonic transducers 101 and 102 alternately functions as a receiver and as a transmitter. Thus, these transducers 101 and 102 are sometimes called “ultrasonic transceivers”. The directions in which the ultrasonic wave is propagated along the propagation path L are indicated by the arrow with L bidirectionally.
In FIG. 9, the flow velocity of the fluid flowing through the tube 52 is supposed to be V, the velocity of the ultrasonic wave in the fluid is supposed to be C, and the angle defined between the direction in which the fluid is flowing (as indicated by the bold arrow) and the direction in which the ultrasonic pulse is propagated (as indicated by the arrow with L) is supposed to be θ. If the ultrasonic transducers 101 and 102 are used as a transmitter and a receiver, respectively, then the following Equation (4) is satisfied:f1=1/t1=(C+V cos θ)/L  (4)where t1 is the sing-around period (i.e., the time it takes for the ultrasonic pulse, radiated from the ultrasonic transducer 101, to reach the ultrasonic transducer 102) and f1 is the sing-around frequency.
Conversely, if the ultrasonic transducers 102 and 101 are used as a transmitter and a receiver, respectively, then the following Equation (5) is satisfied:f2=1/t2=(C−V cos θ)/L  (5)where t2 is the sing-around period and f2 is the sing-around frequency in that situation.
The difference Δf between these two sing-around frequencies is given by the following Equation (6):Δf=f1−f2=2V cos θ/L  (6)
According to Equation (6), the flow velocity V of the fluid can be obtained from the distance L of the ultrasonic wave propagation path and the frequency difference Δf. And the flow rate can be determined by the flow velocity V.
In the conventional ultrasonic generators, the acoustic matching layer thereof is often made of a material with a low density (e.g., a material obtained by solidifying a glass balloon or a plastic balloon with a resin material) to decrease the acoustic impedance thereof. Or the acoustic matching layer may also be formed by a technique of thermally compressing a glass balloon or a technique of foaming a molten material. These methods are disclosed in Japanese Patent No. 2559144, for example.
The conventional acoustic matching layer has its acoustic impedance Z2 decreased by introducing glass balloons, of which the particle sizes are smaller than the wavelength of the ultrasonic wave, into an epoxy resin (i.e., by dispersing air gaps, having too small acoustic impedances to diffuse the ultrasonic wave, in the epoxy resin). Thus, it is imaginable to further decrease the acoustic impedance by increasing the mixture ratio of the glass balloons to the epoxy resin. However, if the mixture ratio of the glass balloons is increased, then the epoxy resin agent with those glass balloons will have an increased viscosity, thus making it hard to mix the glass balloons and the epoxy resin agent together uniformly. For that reason, the mixture ratio of the glass balloons to the epoxy resin agent cannot be increased to beyond a certain limit. Consequently, it is difficult to make an acoustic matching layer with even lower acoustic impedance of the epoxy resin with glass balloons.
Also, the acoustic matching layer made of the conventional epoxy resin with glass balloons has an acoustic impedance of about 1.2×106 kg/s·m2, which is approximately two-thirds of that of an acoustic matching layer made of the epoxy resin only. Thus, the ultrasonic energy transmittance T can be 9/4 times as high as that of the acoustic matching layer made of the epoxy resin only.
Even so, when the ultrasonic energy transmittance T is calculated by the Equation (3) described above on such an acoustic matching layer made of the epoxy resin with glass balloons, it can be seen that T≈3%, which is not sufficient.
Also, the acoustic matching layer included in the conventional ultrasonic transducer for use in an ultrasonic flowmeter is obtained by thermally compressing glass balloons or foaming a molten material as described above. Thus, the medium easily becomes non-uniform due to the damage of the glass spheres under excessive pressures, separation of the glass spheres under insufficient pressures or foaming of the peeled molten material. As a result, a variation in characteristic is created in the same acoustic matching layer and the precision of the equipment also varies unintentionally.
Furthermore, in the manufacturing process of the conventional acoustic matching layer made of the epoxy resin with glass balloons, the cured epoxy resin with the glass balloons is subjected to some machining process such as cutting and/or surface polishing to adjust the sizes and/or thickness thereof to desired values. Thus, the acoustic matching layer may have a thickness that is significantly different from its preferred value, non-uniform thicknesses, or surface unevenness. As a result, the performance of the resultant ultrasonic transducer was not good enough.
On the other hand, the applicant of the present application disclosed in Japanese Patent Application No. 2001-56501 (filed on Feb. 28, 2001) that an acoustic matching layer made of a dry gel exhibits a reduced variation in characteristic as compared with the conventional epoxy resin with the glass balloons.
However, in order to achieve even higher performance for ultrasonic flowmeters, for example, the variation in the characteristic of such an acoustic matching layer made of a dry gel is preferably further reduced.
The present inventors discovered via experiments that even an acoustic matching layer made of a dry gel still exhibited some variation in thickness, which was smaller than the conventional one, though. The present inventors also discovered that if the dry gel was formed by drying a wet gel, the variation in characteristic was caused due to the non-uniformity of the drying process step.